The present invention relates to machine vision processes, and more particularly to a method and apparatus for inspecting solder paste deposited on a substrate using a solder paste printing or dispensing machine. In addition, the present invention relates to methods and apparatuses for controlling solder paste printers and dispensing machines.
In typical surface-mount circuit board manufacturing operations, a stencil printer is used to print solder paste onto a circuit board. Typically, a circuit board having a pattern of pads or some other, usually conductive, surface onto which solder paste will be deposited is automatically fed into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to properly align the circuit board with the stencil or screen of the stencil printer prior to the printing of solder paste onto the circuit board. In some prior art systems, an optical alignment system is used to align the circuit board with the stencil. Examples of optical alignment systems for stencil printers are described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991 to Freeman, and in U.S. Pat. No. Re. 34,615 issued Jan. 31, 1992, also to Freeman, each of which is incorporated herein by reference.
In some prior art systems, the optical alignment system also includes a vision inspection system to inspect a substrate after solder paste has been deposited thereon. These prior art systems generally rely on thresholding techniques to inspect a substrate having solder paste deposited thereon to determine whether the solder paste has been properly applied to conductive pads strategically located on the substrate. As is well known in the art, thresholding is an image segmentation technique used to divide an image into sections based on brightness. The technique segments an image into regions of interest and removes all other regions deemed not essential.
At present, most common techniques for identifying solder paste regions on a printed circuit board primarily consist of single or dual thresholding methods that typically use pixel brightness to identify solder paste regions on a substrate. One drawback of these techniques is that brightness levels of the solder paste and the background often overlap making it difficult, if not impossible, to effectively separate the solder paste images from background information.
Some prior art systems rely primarily on subtraction techniques that require a unique reference image for each inspection. These methods typically compare differences between the reference image (i.e. before) and the current image (after) to detect paste. One drawback of these techniques is that they require relatively significant storage, retrieval and buffer capabilities to accommodate the reference images. Another drawback of these techniques is that they are subject to and are adversely affected by typical variations found between otherwise identical substrates. To avoid this problem, new reference images must be acquired for each substrate to be inspected, but this action can slow the process considerably and may be inappropriate for use in a high speed production environment.
Embodiments of the present invention perform mathematical operations on a digitized image to separate areas of solder paste texture from other dissimilarly textured (non solder paste) areas on a printed circuit board, stencil, wafer, or substrate.
In general, one aspect of the invention features a method of inspecting a substrate having a substance deposited thereon. The steps of the method include depositing the substance onto the substrate; capturing an image of the substrate; detecting variations in texture in the image to determine a location of the substance on the substrate; and comparing the location of the substance with a desired location.
The method can also include the step of comparing an area of the substrate having the substance with a desired area. The substance may include solder paste.
The method can further include the step of detecting variations in solder paste texture by detecting variations independent of the relative brightness of the non-substance background features on the substrate by dividing the image into pixels, and comparing the brightness levels of each pixel with its pixel neighbors.
In the method, a real-time image may be captured and digitized.
In another aspect of the invention, the step of depositing a substance includes the step of printing a substance onto a substrate using a stencil printer. The method further includes the step of providing a feedback signal to adjust the stencil printer. The stencil printer is adjusted based upon a comparison of the location of the substance with the desired location. The feedback signal may also be used to adjust the stencil printer based upon the comparison of an area of the substance with either a desired or predetermined area.
In another aspect, the invention includes a screen printer that dispenses a viscous material at a predetermined location on a substrate. The screen printer includes: a frame; a material dispenser, coupled to the frame, the material dispenser containing a viscous material to be printed on a substrate; a support apparatus, coupled to the frame, that supports the substrate in a printing position located beneath the material dispenser; a camera having a field of view that includes the substrate in a printing position; and a processor in electrical communication with the camera. The processor being programmed to perform texture based recognition on the viscous material printed on the substrate.
In yet another aspect, the invention includes a system for dispensing solder paste at a predetermined location on a substrate. The system includes a dispenser that dispenses material on the substrate; a controller for maintaining the operations of the dispenser; and a processor. The processor is in electrical communication with the controller. The processor is programmed to perform texture based recognition of a solder paste deposit located on the substrate. The processor also determines the quality of the solder paste deposit, and the contours of the solder paste deposit relative to predetermined locations on the substrate. The processor is part of a control loop that comprises the controller in electrical communication with the dispenser.
In still another aspect, the invention includes an inspection system for detecting the presence and location of a substance on a substrate. The inspection system includes a locator that performs location analysis on various pixels in an image of the substance on the substrate. The inspection system also includes a processor that is in electrical communication with the locator. The processor is programmed to perform texture based recognition of the substance located on the substrate, and to determine the quality of the substance and the contours of the substance relative to predetermined locations on the substrate.
In addition, the processor may further include a locator, a tuner, and a texture segmentation filter. The locator performs location analysis on various pixels in an image. The tuner varies brightness and contrast levels in the image and includes an automatic gain, offset, and an exposure management system. The tuner provides aliasing control based on an effective sampling rate. The texture segmentation filter is in electrical communication with the locator and is used to divide the image into texture regions. The texture segmentation filter includes a sharpening operator, a Laplace operator, and a smoothing operator for further improving image quality. The smoothing operator further includes a mechanism for boosting or enhancing the effect of the texture segmentation filter.
Alternatively, the inspection system may also include a first filter that is in electrical communication with the locator wherein the first filter includes a filter for enhancing the image. The inspection system may further include a tuner in electrical communication with the first filter that has an automatic gain, offset, and exposure management system that varies brightness and contrast levels in the image; and a second filter in communication with the tuner that provides anti-aliasing when the image of the substance on the substrate is magnified beyond the effective sampling rate of the process.
In another aspect of the invention, an option is provided to increase the speed of operation of the inspection system. Specifically, a fill-factor option may be selected to produce only a partially processed image of the substrate to be appropriately interpreted as such in subsequent processes.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.